Pan (/ˈpæn/PAN, Greek: Πάν) is the second-innermost moon of Saturn. It is a walnut-shaped small moon about 35 kilometres across and 23 km high that orbits within the Encke Gap in Saturn's A Ring. Pan acts as a ring shepherd and is responsible for keeping the Encke Gap free of ring particles.

It was discovered by Mark R. Showalter in 1990 from analysis of old Voyager 2 probe photos and received the provisional designation S/1981 S 13 because the discovery images dated back to 1981.[3]

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The existence of a moon in the Encke Gap was first predicted by Jeffrey N. Cuzzi and Jeffrey D. Scargle in 1985, based on wavy edges of the gap which indicated a gravitational disturbance.[4] In 1986 Showalter et al. inferred its orbit and mass by modeling its gravitational wake. They arrived at a very precise prediction of 133,603 ± 10 km for the semi-major axis and a mass of 5–10×10−12 Saturn masses, and inferred that there was only a single moon within the Encke gap.[5] The actual semi-major axis differs by 19 km and the actual mass is 8.6×10−12 of Saturn's.

The moon was later found within 1° of the predicted position. The search was undertaken by considering all Voyager 2 images and using a computer calculation to predict whether the moon would be visible under sufficiently favorable conditions in each one. Every qualifying Voyager 2 image with resolution better than ~50 km/pixel shows Pan clearly. In all, it appears in eleven Voyager 2 images.[6][7]

The eccentricity of Pan's orbit causes its distance from Saturn to vary by ~4 km. Its inclination, which would cause it to move up and down, is not distinguishable from zero with present data. The Encke Gap, within which Pan orbits, is about 325 km wide.

Pan in the center of the image, occupying the Encke Gap in Saturn's rings. Its walnut-like shape is clearly visible

Cassini scientists have described Pan as "walnut-shaped"[8] owing to the equatorial ridge, similar to that on Atlas, that is visible in images. The ridge is due to ring material that Pan has swept up from the Encke gap.

Closeup of the Encke Gap. The central ringlet is coincident with Pan's orbit.

The Encke Gap contains a ringlet that is coincident with Pan's orbit, indicating that Pan maintains the particles in horseshoe orbits. A second ringlet is periodically disrupted by Pan similarly to how the F Ring is disturbed by Prometheus.

The moon was named on 16 September 1991,[9] after the mythological Pan, who was (among other things) the god of shepherds. This is a reference to Pan's role as a shepherd moon. It is also designated Saturn XVIII.[10]